There have been numerous attempts in the prior art to provide flame retardant thermoplastics. Typically it has been necessary to heavily fill the plastic or thermoplastic material with additives until the desired degree of flame retardancy had been achieved. However, this offered several disadvantages insofar as a large proportion of additives could normally be expected to detract from the physical properties of the plastic base. Furthermore, it was not unusual to find in a single system large quantities of halogen-containing materials as well as metal compounds.
It will be seen that the present invention provides improved flame retardant compounds which not only require a lower relative proportion of additives, but also avoid the necessity for utilizing organic halides and certain metal compounds such as antimony oxide, which may be undesirable in certain applications.
In U.S. Pat. No. 4,235,978 (Luce et al.) there is disclosed a flame retardant composition comprised of an admixture of a thermoplastic polymer, a flame retardant additive which is either a low molecular weight polymer of a carbonate of a halogenated dihydric phenol, or a combination of such carbonate compounds and an inorganic or organic antimony-containing compound and up to about 5 weight percent of a diorganopolysiloxane gum.
In U.S. Pat. No. 4,209,566 (Betts et al.) there is disclosed a method of enhancing the electrical properties of polymeric electrical insulation which contain a polar halogen compound by treating the polar halogen compound with a heat reactive liquid silicone polymer which is then heated and reacted throughout the halogen compound.
In U.S. Pat. No. 4,247,446 (Betts et al.) there is disclosed a flame resistant composition comprised of a crosslinked polyolefin, decabromodiphenyl ether, silicone gum and dibasic lead phthalate.
In U.S. Pat. No. 4,273,691 (MacLaury et al.) there is disclosed a flame resistant composition comprised of a blend of organic polymer, silicone polymer and a Group IIA metal carboxylate salt containing six to twenty carbon atoms. Copending patent application Ser. No. 344,167 filed Jan. 29, 1982 (Frye et al.), discloses flame retardant compositions which utilize a different range of silicone polymers, namely low viscosity silanol and trimethyl silyl chainstopped polysiloxane fluids.
In U.S. Pat. No. 4,265,801 (Moody et al.) there is disclosed a flame retarded, melt processable polymer composition which is comprised of a blend of non-silicone polymer such as a thermoplastic, a solid, non-elastomeric, mono-organic polysiloxane resin (a so-called MT resin) and a filler.
All of the foregoing patents and applications are hereby incorporated by reference. It will become apparent that the present inventor has herein provided novel flame retardant compositions which exhibit significant improvements over the prior art compositions.
The present invention is based upon the discovery that efficient flame retardant plastics can be provided by combining in the correct proportions certain metal soaps (e.g. magnesium stearate) with a mixture of certain silicone resins such as a polytriorganosilyl silicate and a polydiorganosiloxane polymer. For example, adding this combination to a polypropylene homopolymer can enable it to exhibit properties meeting the requirements of the well known Underwriter's Laboratories UL-94 vertical burn test. Furthermore, exceptional flame retardancy can be achieved in certain compositions of the present invention without the use of a halogen or antimony oxide; and certain optimized formulations utilizing the compositions of the present invention will enable those skilled in the art to achieve superior V-0 rated plastics when relatively small amounts of certain halogen-containing compounds are also utilized.
This invention differs from the prior art in several respects, notably in that it requires the use of a combination of polyorganosiloxane silicone and a silicone resin, thereby achieving a synergistic flame retardant effect which is not taught by the prior art. The degree of flame retardancy is significantly greater and it was quite surprising that improved flame retardancy could be achieved by the addition of certain specified amounts of silicone resin in combination with previously known flame retardant compositions such as those described by MacLaury et al. or Frye et al.
The metal/silicone/silicone resin flame retardant of the present invention provides at least three significant advantages over the commonly used halogen/antimony oxide systems for thermoplastics: (1) the metal/silicone/silicone resin system is free from the acidic and perhaps toxic by-products released by halogen and antimony upon burning, (2) the typical concentration of the metal/silicone/silicone resin additive composition needed for a V-1 type flammability formulation is in the range of 20% and hence the mechanical properties of a flame retarded thermoplastic containing the additive composition are not altered to as great a degree as the same thermoplastic in which other flame retardant additives have been incorporated in higher concentrations to obtain equivalent flame retardant properties, and (3) the presence of silicone provides an improvement in the processability of the polymer, thereby adding an unexpected beneficial side effect. Additionally, thermoplastic products containing the flame retardant additives of the present invention when prepared according to the process of the present invention exhibit a higher level of gloss than finished thermoplastic products containing conventional flame retardant additives. In addition, the relatively low amount of metal/silicon silicone resin necessary for good flame retardance permit typical flame retardant formulations of the present invention to be less expensive than conventional flame retardant formulations based on the same polymer system. Polypropylene, for example, is a large volume thermoplastic having many desirable properties such as solvent and moisture resistance but also having high flammability. The invention herein described can provide a desirable V-1 grade of flame retardant polypropylene using only about 20% total flame retardant additives by weight and omitting antimony oxide and halogen. By contrast, commercially available flame retardant grades of polypropylene use 30-50% by weight of organic halide and an antimony oxide synergist (Sb.sub.2 O.sub.3). These high additive loadings increase the cost of the formulation and decrease its tensile strength and other physical properties. The increase in cost and change in properties make commercial flame retardant polypropylene unsuitable for many applications. Additionally, the presence of antimony oxide in such compositions raises concerns about (1) toxicity and carcinogenicity, (2) char afterglow, and (3) product appearance. Further Sb.sub.2 O.sub.3 can cause a brightening effect which will require the use of additional pigment in applications with critical appearance specifications.
In the present invention antimony oxide has been replaced by silicone, a silicone resin such as an MQ resin, and a metal salt such as an alkaline-earth metal carboxylate additive such as magnesium stearate. These additives are non-toxic, do not support char afterglow, can be used at lower levels, and may contribute to other beneficial properties such as processability, lubricity, mold release, and gloss. The smaller proportion of flame retardant additives required in this invention is expected to have less of a detrimental effect on the physical properties of the thermoplastic than do the heavier loadings of commercial formulations. Furthermore, it is expected that the improved physical properties will in turn facilitate expanded uses for flame retarded thermoplastics.
It is therefore an object of the present invention to provide improved flame retardant additives for plastics which utilize a novel combination of silicone and silicone resin and metal soap, but which do not require the use of either organic halide or antimony oxide.
There are also provided flame retarded thermoplastic compositions and articles made therefrom which contain a combination of silicone, silicone resin and metal soap.
There are also provided processes for achieving each of the aforesaid objectives.
These and other objects will become apparent to those skilled in the art upon consideration of the present specification and claims.